Magnetic disk apparatuses include magnetic disks on which data is stored. When reading data from or write data to a magnetic disk, a magnetic head is moved to a specific track, i.e. make the magnetic disk on-track. This operation is know as a seek operation.
If the magnetic disk is faulty, high-frequency oscillation components that have a frequency which is more than half of a servo interruption frequency, are generated in the oscillation frequency of the magnetic head. In the on-track status, these components cannot be monitored. A means to effectively solve this problem has been sought in the past.
FIG. 18 is a perspective view of a conventional magnetic disk apparatus 10. The magnetic disk apparatus 10 includes a chassis 11 and a cover 12. The chassis 11 and the cover 12 form an air-tight container within which are included a hard disk assembly 14, a print circuit board 22, and a connector 23. All circuits are mounted on the print circuit board 22. The connector 23 electrically connects the parts of the hard disk assembly 14 and the print circuit board 22.
The chassis 11 accommodates a plurality of magnetic disks 151 through 15n, a spindle motor 16, magnetic heads 171, through 17n, a carriage 18, a flexible print circuit sheet 19, a head integrated circuit (IC) 20, and a gasket 13. One magnetic head is provided for each magnetic disk at the tip of an arm 21. The cover 11 is closed air-tightly to the chassis 11 because of the gasket 13.
The magnetic disks are stacked above one another at a fixed interval. The spindle motor 16 drives the magnetic disks at a high speed. The carriage 18 supports the magnetic heads through the arm 21.
FIG. 19 is a block diagram of servo circuit parts of the conventional magnetic disk apparatus 10. The parts in FIG. 19 that are identical to those in FIG. 18 are assigned identical reference numerals. The magnetic disk 151 is partitioned into, for example, ten servo sectors SP1 through SP10. One data sector is provided between two servo sectors. Thus, there are ten data sectors D1 through D10.
Servo patterns that recognize the position of the magnetic head 171 are stored in the servo sectors. On the other hand, data are stored in the data sectors. A plurality of concentric cylinders exist in the magnetic disk 151 through 15n. 
The magnetic head 171, which is located close to the top of the magnetic disk 151, includes a head core and a coil that is wound around the head core.
When writing, an electric current is passed through the coil of the magnetic head 171. A magnetic field is generated because of the and the data is written on the data sectors. On the other hand, when reading, the magnetic head 171 detects, as magnetically regenerated voltage, the servo patterns stored in the servo sectors and the data stored in the data sectors.
The head IC 20 includes a write amplifier (not shown) and a preamplifier (not shown). The write amplifier switches the polarity of the recording current to be supplied to the magnetic head 171 in accordance with the write data. The preamplifier amplifies the regenerated voltage (read signals) detected by the magnetic head 171.
A read channel (RDC) 30 includes a circuit for writing the write data to the magnetic disk 151 and a circuit for reading the read data or the servo pattern from the magnetic disk 151. The read channel 30 further includes a parallel/serial converting circuit that converts parallel write data into serial data, a synthesizer circuit that generates timing signals for each part of the apparatus by stepping up the frequency of an oscillating circuit that, in turn, employs a crystal oscillator.
After the servo pattern input via the read channel 30 is modulated by peak-hold or integration, a digital servo controller 31 controls (servo control), based on the modulated servo pattern, the position of the magnetic head 171, by controlling the driving currents of a voice coil motor (VCM) 32 and the spindle motor 16 (see FIG. 18). The voice coil motor 32 is a driving source of the seek operation which involves radially shifting and positioning the magnetic head 171 on a specific cylinder by driving the cylinder 18 using the driving currents.
When the magnetic disk 151 is being driven and the magnetic head 171 is in the on-track status, data reading/writing and servo pattern reading are repeated in an alternating manner according to the sequence of the servo sectors and the data sectors.
When the reading or writing operation of data is interrupted and the servo pattern is read, it is called a servo interruption. The period for which the servo interruption takes place is called a servo interruption period. The frequency during the servo interruption is called a servo interruption frequency.
FIG. 20 illustrates the servo interruption. The saw-tooth waveform W1 is a waveform of a frequency f1. The points t1, t2, . . . , indicated by hollow circles, are points in time when servo interruption occurs in each servo interruption period T1. The vertical axis represents the position of the magnetic head 171. In other words, when the position is zero (0), it indicates that the magnetic head 171 is on the target cylinder (that is, an on-track status). When the position is not zero, it indicates that the magnetic head 171 has shifted from the target cylinder and requires a position correction.
The digital servo controller 31 shown in FIG. 19 demodulates the servo pattern that is read by the magnetic head 171 and input through the read channel 30 in each servo interruption period T1, and recognizes the position of the magnetic head 171. If the magnetic head 171 is found to be shifted, the digital servo controller 31 changes the drive current supplied to the voice coil motor 32 and carries out the servo control in order to make the status of the magnetic head 171 on-track.
The servo pattern stored in each of the servo sectors SP1 through SP10 of the magnetic disk 151 is explained next with reference to FIG. 21. A servo pattern 100 comprises a servo preamble 110, a servo mark 120, a gray code 130, and a burst 140. The servo preamble 110 corresponds to a reference signal of a servo gain.
The servo mark 120 produces the servo interruption. The gray code 130 and the burst 140 represent the cylinder of the magnetic disk 151. In other words, the gray code 130 represents the whole number part of the cylinder and the burst 140 represents the fraction part of the cylinder. For example, if the cylinder is represented by 1000.0001, the gray code 130 represents 1000 (whole number) and the burst 140 represents 0.0001 (fraction).
FIG. 22 illustrates the burst 140 and all types of signals. A plurality of patterns of the burst 140 exist between the cylinders cy 1.0000 and cy 1.0004 in the magnetic disk 151. For instance, when the burst 140 of the cylinder cy 1.0000 is read by the magnetic head 171, four types of signals having a triangular waveform, namely, signal PosA, signal PosB, signal PosC, and signal PosD, are obtained. The signals PosA and PosB have a reverse phase relation. Similarly, the signals PosC and PosD have a reverse phase relation. The signals PosA and PosC have a phase difference of π/2 between them. Similarly, the signals PosB and PosD have a phase difference of π/2 between them.
When the burst 140 is read by the magnetic head 171, a position deviation signal A illustrated in FIG. 23 is generated from the signals PosA, PosB, PosC, and PosD in a modulator (not shown). The position deviation signal A represents the offset amount from the center of a track and comprises opposing signals PosN and PosQ.
As shown in FIG. 22, the signal Pos N is obtained by subtracting the signal PosB from the signal PosA. Similarly, the signal PosQ is obtained by subtracting the signal PosD from the signal PosC. The modulator selects the opposing signals PosN and PosQ one after another and generates the position deviation signal A. Conventionally, the modulator carries out a linear correction by multiplying a burst modulation value with a constant correction value so that the burst modulation value coincides with the meeting point of the signals PosN and PosQ, as illustrated in FIG. 23.
When the magnetic head 171 is positioned on the target track, the servo control takes place in such a way that, depending on the current instruction value corresponding to the position deviation signal A, the magnetic head is positioned at the center of the target track.
If the magnetic head 171 is in the on-track status, disturbances of various kinds occur, which may cause the magnetic head 171 to vibrate. These disturbances may be vibration due to rotation of the magnetic disk 151, air pressure, shake due to the burst, jitter due to the drive current, or resonance of the Kashime junction between the magnetic head 171 and the arm 21.
When an oscillation waveform is obtained during disturbances, it is observed that high-frequency oscillation components that are included in the position signals obtained from the servo pattern and that have a frequency exceeding half of the servo interruption frequency (1/T1), can pose a problem.
According to Shannon's theorem, if the high-frequency oscillation components that have a frequency which is more than half of the servo interruption frequency (sampling frequency) are included, these high-frequency oscillation components cannot be monitored and thus influence the positioning accuracy of the magnetic head 171.
For instance, when the servo interruption frequency in FIG. 20 is 8.64 kHz and the resonance frequency (high-frequency oscillation components) at the Kashime junction due to the disturbances is 8.6 kHz, then according to Shannon's theorem, the high-frequency oscillation components cannot be monitored.
It has already been described that a servo control, which conventionally involves a linear correction process in which the burst modulation value is multiplied by a constant correction value, as illustrated in FIG. 23, is carried out.
If the magnetic head 171 has a non-linear head sensitivity characteristic, a non-linear position deviation signal B illustrated in FIG. 23 would be ideal. However, since the magnetic head 171 is positioned according to the position deviation signal A, the precision of positioning decreases by the difference between the position deviation signal A and the position deviation signal B.
If the magnetic head 171 has a non-linear head sensitivity characteristic, there is an another problem that if a zero-cross frequency, to which disturbance of minute amplitude is added, is introduced in the design, an open loop gain does not become 0 dB at the center of the track (=0), as illustrated by a characteristic line D in FIG. 24. The open loop gain varies, being high at the center of the track and low in offset positions.